The present invention relates to a membrane-type reaction vessel which facilitates oxygen reactions using an oxygen-permeable membrane that allows oxygen, from oxygen-containing gas, to pass therethrough selectively.
An oxygen-permeable membrane is a semipermeable membrane which allows oxygen to pass therethrough selectively depending upon the partial pressure of oxygen on opposite sides of the membrane. The membrane typically consists of a solid electrolyte that conducts oxygen ions, and examples of such materials include yttrium-stabilized zirconia (YSZ), calcia-stabilized zirconia, and bismuth oxide.
It is also possible to create an oxygen-permeable membrane with a mixed conductor which conducts both oxygen ions and electrons, instead of a solid electrolyte which conducts ions only. For example, YSZ containing titanium and compounds with a Perovskite structure, such as Laxe2x80x94Srxe2x80x94Coxe2x80x94O, Laxe2x80x94Srxe2x80x94Coxe2x80x94Fexe2x80x94O and Srxe2x80x94Coxe2x80x94O, are known.
The use of such oxygen-permeable membranes for oxygen isolation equipment or chemical reaction equipment has become popular recently. For example, U.S. Pat. No. 5,306,411 discloses an electrochemical reactor including a solid membrane made of mixed metal oxides having a Perovskite structure (i.e., a mixed conductor). The solid membrane is used as an electrochemical transportation mechanism for moving oxygen from an oxygen-containing gas to a source gas, to remove oxygen from oxygen-containing compounds, to allow for aromatic compound substitution, and/or to allow for partial oxidation reactions of methane and ethane.
Japanese Patent Application No. JP-A-7-138191 also discloses a membrane reactor which facilitates an oxidative coupling reaction of methane, by using Perovskite-structured oxides such as Srxe2x80x94Coxe2x80x94Lixe2x80x94O and Srxe2x80x94Coxe2x80x94Mgxe2x80x94O as membrane compounds.
The reaction vessels described above provide superior selectivity of reaction in comparison to vapor phase reaction vessels, which were commonly used to facilitate such reactions. There are, however, problems that need to be solved so as to allow safe operation of such vessels.
FIGS. 2A and 2B show one example of a reaction vessel known in the prior art. The reaction vessel 21 includes a vessel wall 22 defining an oxygen-containing gas chamber 28 and a source gas chamber 29. These two chambers are separated hermetically by an oxygen-permeable membrane 23. Oxygen-containing gas inlet 24 and outlet 25 allow gas to flow through chamber 28. Source gas inlet 26 and outlet 27 allow gas to flow through chamber 29.
There is a concern that, in the case of damage (e.g., cracking) to the oxygen-permeable membrane 23, a large volume of oxygen-containing gas, which is confined in oxygen-containing gas chamber 28 only by oxygen-permeable membrane 23, will flow into source gas chamber 29, or conversely, that a large volume of source gas will flow into oxygen-containing gas chamber 28. In such cases of large gas volume, there is a danger that the two gases mixed together will react rapidly and result in explosion.
It is an object of the present invention to provide a reaction vessel that is safer to operate and has a lower possibility of explosion even in the event of cracking of or damage to the oxygen-permeable membrane.
In accordance with one embodiment of the present invention, the membrane-type reaction vessel is divided into an oxygen-gas containing chamber and a source gas chamber via an oxygen-permeable membrane which is selectively permeable to oxygen. The vessel is designed so as to allow oxygen from the oxygen-containing gas to pass through the oxygen-permeable membrane to contact and react with source gas in the source gas chamber. The volume ratio between the source gas chamber and the oxygen-containing gas chamber is designed to be outside the range within which the two gases, if combined directly, would result in explosion. Preferably, the volume ratio between the two gas chambers is also designed to be outside the range within which an explosion would result from directly combining the oxygen-containing gas and reaction products resulting from the reaction between oxygen that has already permeated through the oxygen-permeable membrane and the source gas.
In a more preferred embodiment of the present invention, the volume ratio of the oxygen-containing gas chamber with respect to the total volume (VT) of the reaction vessel (i.e., the volume (VO) of the oxygen-containing gas chamber and the volume (VS) of the source gas chamber) should be less than 50%, more preferably less than 30%, and most preferably less than 10%.
In accordance with another preferred embodiment of the present invention, a filler is disposed in the oxygen-containing gas chamber and/or the source gas chamber to adjust the volume of those chambers relative to each other. The filler preferably is one of a metal, ceramic or glass, all of which are inert with respect to the oxygen-containing gas, the source gas and reaction products resulting from reactions between oxygen supplied from the oxygen-containing gas and the source gas.